LED Resistor Calculator

Written byOleh Soloninupdated 25 September 2026

Supply voltage, V

LED forward voltage, V

The exact figure is on the LED's datasheet. Wikipedia's table gives red 1.63–2.03 V, blue 2.48–3.7 V, white 2.7–3.5 V.

LED current, mA

Number of LEDs

Wiring

Result

Nearest standard value150 ohm
Exact resistance150 ohm
Dissipation, W0.06
Resistor rating
0.06 W is dissipated — pick a resistor rated at least that, for example 0.125 W
Total current, mA20
Current with this value, mA20
Resistance, Ω150
WorkingR = (5 − 2) ÷ 0.02 = 150 ohm; P = 3 × 0.02 = 0.06 W

An LED needs its current limited — by a resistor or a driver

Sizes the series resistor from supply voltage, LED forward drop and working current, and picks the nearest standard value.

An LED's current is set by a series resistor. The resistor takes up the difference between the supply voltage and the LED's forward voltage, so by Ohm's law R = (V supply − V forward) ÷ I.

The forward voltage depends on the colour: the table in Wikipedia's "Light-emitting diode" article gives 1.63–2.03 V for red, 2.48–3.7 V for blue and 2.7–3.5 V for white. The exact figure and the rated current are on the LED's datasheet.

How to use

Enter the supply voltage, the LED's forward voltage and its operating current from the datasheet.
Set the number of LEDs and whether they are in series or in parallel.
Read the exact resistance, the next E24 value up, the current with that value and the power the resistor dissipates.

Resistors for common circuits

Each row is a separate run of the calculator. The value is the nearest E24 value not below the exact one.

All rows come from the calculator on this page.
Supply	LEDs	Current	Exact	E24 value	Dissipated
5 V	1 × 2 V	20 mA	150 Ω	150 Ω	0.06 W
5 V	1 × 3.2 V	20 mA	90 Ω	91 Ω	0.036 W
12 V	1 × 2 V	20 mA	500 Ω	510 Ω	0.2 W
12 V	3 × 3.2 V in series	20 mA	120 Ω	120 Ω	0.048 W
12 V	5 × 2 V in series	20 mA	100 Ω	100 Ω	0.04 W
24 V	6 × 3.2 V in series	20 mA	240 Ω	240 Ω	0.096 W
3.3 V	1 × 2 V	10 mA	130 Ω	130 Ω	0.013 W

Series or parallel

In a series string the forward voltages add up and the current is shared: three white LEDs at 3.2 V on 12 V need 120 Ω. The supply has to cover the sum of the forward voltages, and the calculator warns when it does not.

Parallel LEDs on one shared resistor split the current evenly only when their forward voltages are closely matched, as Wikipedia's "LED circuit" article notes. For a parallel circuit the calculator gives both the shared resistor and one per branch: for three red LEDs on 12 V, 180 Ω shared or 510 Ω each.

What this calculator shows

The exact resistance and the nearest standard E24 value
The power the resistor will dissipate and the rating it needs
Total circuit current when several LEDs are involved

What to keep in mind

Forward voltage and current come from the datasheet; the calculator treats them as fixed.
The resistor value is the nearest E24 value (IEC 60063) not below the exact one.

FAQs

What resistor does an LED need on 5 volts?

For a red LED with a 2 V forward voltage at 20 mA, (5 − 2) ÷ 0.02 = 150 Ω. A white one at 3.2 V comes to 90 Ω, and the next E24 value up is 91 Ω.

What resistor does an LED need on 12 volts?

For one red LED at 2 V and 20 mA, 500 Ω; from the E24 series take 510 Ω, which gives 19.6 mA.

Why does the calculator round up rather than to the nearest value?

At the nominal values a larger resistor gives slightly less current than planned, and a smaller one would push it above the target. Tolerances still count: a 5% resistor at its low end, a higher supply voltage or a lower forward voltage raise the current, so near the LED's limit check those extremes too.

What wattage should the resistor be?

At least what it dissipates: P = (V supply − V forward) × I. On 12 V with a red LED at 20 mA that is 0.2 W — a 0.125 W resistor is too small, a 0.25 W one or larger will do.

Can I run LEDs in parallel on one resistor?

You can, but the current splits evenly only if the LEDs' forward voltages are closely matched — Wikipedia's "LED circuit" warns about this. A resistor in each branch is safer, and the calculator shows that option too.

Where do I find an LED's forward voltage?

On its datasheet. As a guide, Wikipedia's table gives red 1.63–2.03 V, yellow 2.10–2.18 V, blue 2.48–3.7 V and white 2.7–3.5 V.

Worked example

An LED on USB power

Input: 5 V, 2 V, 20 mA, 1 LED

Output: A 150 Ω resistor dissipating 0.06 W

Note: A 0.125 W resistor covers 0.06 W with room to spare.

Useful links

All tools in Electrical

LED Resistor Calculator

Result

Related tools

Ohm's Law Calculator

Resistor Color Code

Battery Life Calculator

Useful links